1 Chapter 9 Hypothesis Testing Developing Null and Alternative Hypotheses Type I and Type II Errors One-Tailed Tests About a Population Mean: Large-Sample Case Two-Tailed Tests About a Population Mean: Large-Sample Case Tests About a Population Mean: Small-Sample Case Tests About a Population Proportion

2 Developing Null and Alternative Hypotheses Hypothesis testing can be used to determine whether a statement about the value of a population parameter should or should not be rejected. The null hypothesis, denoted by H 0, is a tentative assumption about a population parameter. The alternative hypothesis, denoted by H a, is the opposite of what is stated in the null hypothesis.

3 Testing Research Hypotheses –Hypothesis testing is proof by contradiction. –The research hypothesis should be expressed as the alternative hypothesis. –The conclusion that the research hypothesis is true comes from sample data that contradict the null hypothesis. Developing Null and Alternative Hypotheses

4 Testing the Validity of a Claim –Manufacturers’ claims are usually given the benefit of the doubt and stated as the null hypothesis. –The conclusion that the claim is false comes from sample data that contradict the null hypothesis.

5 Testing in Decision-Making Situations –A decision maker might have to choose between two courses of action, one associated with the null hypothesis and another associated with the alternative hypothesis. –Example: Accepting a shipment of goods from a supplier or returning the shipment of goods to the supplier. Developing Null and Alternative Hypotheses

6 The equality part of the hypotheses always appears in the null hypothesis. In general, a hypothesis test about the value of a population mean  must take one of the following three forms (where  0 is the hypothesized value of the population mean). H 0 :  >  0 H 0 :  <  0 H 0 :  =  0 H a :   0 H a :   0 A Summary of Forms for Null and Alternative Hypotheses about a Population Mean One-tailedOne-tailedTwo-tailed

7 Example: Metro EMS Null and Alternative Hypotheses A major west coast city provides one of the most comprehensive emergency medical services in the world. Operating in a multiple hospital system with approximately ____ mobile medical units, the service goal is to respond to medical emergencies with a mean time of _____ minutes or less. The director of medical services wants to formulate a hypothesis test that could use a sample of emergency response times to determine whether or not the service goal of 12 minutes or less is being achieved.

8 Example: Metro EMS Null and Alternative Hypotheses Hypotheses Conclusion and Action H 0 :  The emergency service is meeting the response goal; no follow-up action is necessary. H a :  The emergency service is not meeting the response goal; appropriate follow-up action is necessary. Where:  = mean response time for the population of medical emergency requests.

9 Type I and Type II Errors Since hypothesis tests are based on sample data, we must allow for the possibility of errors. A Type is rejecting H 0 when it is true. The person conducting the hypothesis test specifies the maximum allowable probability of making a Type I error, denoted by  and called the level of significance.

10 Type I and Type II Errors o A Type is accepting H 0 when it is false. o Generally, we cannot control for the probability of making a Type II error, denoted by . o Statistician avoids the risk of making a Type II error by using “do not reject H 0 ” and not “accept H 0 ”.

11 Type I and Type II Errors Population Condition H 0 True H a True Conclusion (  ) (  ) Accept H 0 Correct Type II (Conclude  Conclusion Error Reject H 0 Type I Correct (Conclude  rror Conclusion Example: Metro EMS

12 Using the Test Statistic n The test statistic z has a standard normal probability distribution. n We can use the standard normal probability distribution table to find the z -value with an area of  in the lower (or upper) tail of the distribution. n The value of the test statistic that established the boundary of the rejection region is called the critical value for the test. n The rejection rule is: Lower tail: Reject H 0 if z < z . Upper tail: Reject H 0 if z > z .

13 Using the p -Value n The p -value is the probability of obtaining a sample result that is at least as unlikely as what is observed. n The p -value can be used to make the decision in a hypothesis test by noting that: if the p -value is less than the level of significance , the value of the test statistic is in the rejection region. if the p -value is greater than or equal to , the value of the test statistic is not in the rejection region. n Reject H 0 if the p -value < .

14 Steps of Hypothesis Testing 1.Determine the null and alternative hypotheses. 2.Specify the level of significance . 3.Select the test statistic that will be used to test the hypothesis. Using the Test Statistic 4.Use  to determine the critical value for the test statistic and state the rejection rule for H 0. 5.Collect the sample data and compute the value of the test statistic. 6.Use the value of the test statistic and the rejection rule to determine whether to reject H 0.

15 Steps of Hypothesis Testing Using the p -Value 4.Collect the sample data and compute the value of the test statistic. 5.Use the value of the test statistic to compute the p - value. 6.Reject H 0 if p -value < .

16 n Hypotheses H 0 :   or H 0 :   H a :   H a :   n Test Statistic  Known  Unknown n Rejection Rule Reject H 0 if z > z  Reject H 0 if z < - z  One-Tailed Tests about a Population Mean: Large-Sample Case ( n > 30)

17 Example: Metro EMS One-Tailed Test about a Population Mean: Large n Let  = P (Type I Error) = __________ Sampling distribution of (assuming H 0 is true and  = 12) Sampling distribution of (assuming H 0 is true and  = 12)  12 c c Reject H 0 Do Not Reject H  (Critical value)

18 Example: Metro EMS One-Tailed Test about a Population Mean: Large n Let n = 40, = minutes, s = 3.2 minutes (The sample standard deviation s can be used to estimate the population standard deviation .) Since 2.47 > ________, we reject H 0.

19 Example: Metro EMS n One-Tailed Test about a Population Mean: Large n Conclusion : We are 95% confident that Metro EMS is not meeting the response goal of 12 minutes; appropriate action should be taken to improve service.

20 Using the p -value to Test the Hypothesis Recall that z = _____ for = _____. Then p -value = Since p -value < , that is.0068 <.05, we reject H 0. Example: Metro EMS Do Not Reject H 0 Reject H 0 z z 2.47 p -value 

21 Hypotheses H 0 :   H a :   Test Statistic  Known  Unknown Rejection Rule Reject H 0 if | z | > z  Two-Tailed Tests about a Population Mean: Large-Sample Case ( n > 30)

22 Example: Glow Toothpaste Two-Tailed Tests about a Population Mean: Large n The production line for Glow toothpaste is designed to fill tubes of toothpaste with a mean weight of 6 ounces. Periodically, a sample of 30 tubes will be selected in order to check the filling process. Quality assurance procedures call for the continuation of the filling process if the sample results are consistent with the assumption that the mean filling weight for the population of toothpaste tubes is 6 ounces; otherwise the filling process will be stopped and adjusted.

23 Example: Glow Toothpaste Two-Tailed Tests about a Population Mean: Large n A hypothesis test about the population mean can be used to help determine when the filling process should continue operating and when it should be stopped and corrected. –Hypotheses H 0 :    H a :  –Rejection Rule  ssuming a.05 level of significance, Reject H 0 if z 1.96

24 Sampling distribution of (assuming H 0 is true and  = 6) Example: Glow Toothpaste Two-Tailed Test about a Population Mean: Large n  Reject H 0 Do Not Reject H 0 z z Reject H 

25 Two-Tailed Test about a Population Mean: Large n Assume that a sample of 30 toothpaste tubes provides a sample mean of 6.1 ounces and standard deviation of 0.2 ounces. Let n = 30, = 6.1 ounces, s =.2 ounces Since 2.74 > 1.96, we reject H 0. Example: Glow Toothpaste

26 Example: Glow Toothpaste Two-Tailed Test about a Population Mean: Large n Conclusion : We are 95% confident that the mean filling weight of the toothpaste tubes is not 6 ounces. The filling process should be stopped and the filling mechanism adjusted.

27 Example: Glow Toothpaste Using the p -Value for a Two-Tailed Hypothesis Test Suppose we define the p -value for a two-tailed test as double the area found in the tail of the distribution. With z = ______, the standard normal probability table shows there is a = ______ probability of a difference larger than.1 in the upper tail of the distribution. Considering the same probability of a larger difference in the lower tail of the distribution, we have p -value = 2(.0031) =.0062 The p -value.0062 is less than  =.05, so H 0 is rejected.

28 Confidence Interval Approach to a Two-Tailed Test about a Population Mean Select a simple random sample from the population and use the value of the sample mean to develop the confidence interval for the population mean . If the confidence interval contains the hypothesized value  0, do not reject H 0. Otherwise, reject H 0.

29 Example: Glow Toothpaste Confidence Interval Approach to a Two-Tailed Hypothesis Test The 95% confidence interval for  is or to ________ Since the hypothesized value for the population mean,  0 = 6, is not in this interval, the hypothesis- testing conclusion is that the null hypothesis, H 0 :  = 6, can be rejected.

30 Test Statistic  Known  Unknown This test statistic has a t distribution with n - 1 degrees of freedom. Tests about a Population Mean: Small-Sample Case ( n < 30)

31 n Rejection Rule H 0 :   Reject H 0 if t > t  H 0 :   Reject H 0 if t < - t  H 0 :   Reject H 0 if | t | > t  Tests about a Population Mean: Small-Sample Case ( n < 30)

32 p -Values and the t Distribution The format of the t distribution table provided in most statistics textbooks does not have sufficient detail to determine the exact p -value for a hypothesis test. However, we can still use the t distribution table to identify a range for the p -value. An advantage of computer software packages is that the computer output will provide the p -value for the t distribution.

33 Example: Highway Patrol One-Tailed Test about a Population Mean: Small n A State Highway Patrol periodically samples vehicle speeds at various locations on a particular roadway. The sample of vehicle speeds is used to test the hypothesis H 0 :  < 65. The locations where H 0 is rejected are deemed the best locations for radar traps. At Location F, a sample of 16 vehicles shows a mean speed of _____ mph with a standard deviation of _____ mph. Use an  =.05 to test the hypothesis.

34 Example: Highway Patrol One-Tailed Test about a Population Mean: Small n  Reject H 0 Do Not Reject H 0 (Critical value) t

35 Example: Highway Patrol One-Tailed Test about a Population Mean: Small n Let n = 16, = 68.2 mph, s = 3.8 mph  =.05, d.f. = 16-1 = 15, t  = Since 3.37 > 1.753, we reject H 0. Conclusion : We are 95% confident that the mean speed of vehicles at Location F is greater than 65 mph. Location F is a good candidate for a radar trap.

36  known ? Summary of Test Statistics to be Used in a Hypothesis Test about a Population Mean n > 30 ? Popul. approx.normal ?  known ? Use s to estimate  Use s to Estimate  Increase n to > 30 Yes Yes Yes Yes No No No No

37 The equality part of the hypotheses always appears in the null hypothesis. In general, a hypothesis test about the value of a population proportion p must take one of the following three forms (where p 0 is the hypothesized value of the population proportion). H 0 : p > p 0 H 0 : p < p 0 H 0 : p = p 0 H a : p p 0 H a : p p 0 A Summary of Forms for Null and Alternative Hypotheses about a Population Proportion One-tailedOne-tailedTwo-tailed

38 Tests about a Population Proportion: Large-Sample Case ( np > 5 and n (1 - p ) > 5) Test Statistic where:

39 n Rejection Rule H 0 : p  p  Reject H 0 if z > z  H 0 : p  p  Reject H 0 if z < -z  H 0 : p  p  Reject H 0 if |z| > z  Tests about a Population Proportion: Large-Sample Case ( np > 5 and n (1 - p ) > 5)

40 Example: NSC Two-Tailed Test about a Population Proportion: Large n For a Christmas and New Year’s week, the National Safety Council estimated that ______ people would be killed and 25,000 injured on the nation’s roads. The NSC claimed that _____% of the accidents would be caused by drunk driving. A sample of 120 accidents showed that 67 were caused by drunk driving. Use these data to test the NSC’s claim with  = 0.05.

41 Example: NSC Two-Tailed Test about a Population Proportion: Large n –Hypothesis H 0 : p =.5 H a : p.5 –Test Statistic

42 Example: NSC Two-Tailed Test about a Population Proportion: Large n –Rejection Rule Reject H 0 if z 1.96 –Conclusion Do not reject H 0. For z = ______, the p -value is.201. If we reject H 0, we exceed the maximum allowed risk of committing a Type I error ( p -value >.050).

43 End of Chapter 9